Condensers of the above described type, are arranged at the so-called cold end of prime movers and have the purpose of providing a greater pressure gradient and heat gradient by generating the highest possible vacuum in, for example, a steam turbine.
In condensers in which the water box is joined via flanges to the tube plate and the condenser shell, the following problems arise:
machining of the extremely large flanges for the modern large condensers on site turns out to be very expensive;
there is a danger in principle of air being able to penetrate into the steam space of the condenser through the large flanges;
leaking flanges can be sealed at a later date only by very rough and ready methods and with great difficulty.
In a completely welded structure, the steel sheet walls of the water boxes are welded to the condenser shell and the tube plates of steel sheet are as a rule welded into the water boxes. This gives rise to the following problems:
the tube plate must be provided with a rust-resisting plating on the water side;
the required protective lining of the water box is drawn over a part of the plated tube plate and becomes very susceptible to damage, in particular in the zone of the tube plate/shell joint;
if the tubes are welded in, there is a risk, due to the axial tube forces during operation, of the plating being detached in the perforated zone of the tube plate;
if, however, the tubes are only rolled in, cooling water due to leakages can, through the plating, reach the tube plate, which is not resistant to sea water, and can cause efflorescent rust thereon.
Power station operators nowadays demand extreme leak-tightness against an irruption of cooling water into condensers. The permissible leakage rates are almost unmeasurable, and this has the result that the hitherto used technique of rolling the tubes in is supplemented by welding the tubes in. In addition, extremely corrosion resistant titanium tubes are used nowadays.
In the case of the flanged joint mentioned, it is then possible to roll and/or weld the titanium tubes also into titanium tube plates. This is obvious in particular for the reason that titanium can virtually only be welded to titanium. For bolting the titanium tube plate to the flanges of both the water box shell and the condenser shell, appropriate gaskets must be provided. The rubber layer of the protective lining, which was required in any case, was therefore arranged between the water box shell and the tube plate, whilst a soft gasket was inserted between the tube plate and the flange of the condenser shell. After a prolonged operating period, however, such a solution can lead to an irruption of both cooling water and air into the steam space, since the gaskets are very highly stressed due to the different expansions of the tubes and the condenser shell.
In the welded structure, and if titanium tubes are used, the plating must also consist of titanium for the reasons given. Due to the heat stresses which occur, however, there is a risk--even if only slight--of the plating becoming detached. Since, in particular in modern nuclear plants which make extremely stringent demands on the purity of the feed water, this is completely unthinkable, the power station operators demand absolutely safe solutions. With respect to corrosion and leak-tightness, only titanium tube plates can therefore be considered next to titanium tubes.